1. Field of the Invention
The present invention relates to a Levenson type phase shift mask used for manufacturing a semiconductor element such as an LSI and to a manufacturing method thereof.
2. Description of the Related Art
Recently, a high resolution quality has been demanded even in a projection exposure device in connection with increasing density and miniaturization of semiconductor elements. Thus, in the field of photomasks, there is a phase shift method proposed by Levenson et al. of IBM Corporation in 1982 as a technique for improving the resolution quality of a transcription pattern. The principle of the phase shift method is as follows: a phase shift portion (shifter opening) is provided in one opening so that the phase of transmitted light which has passed an adjacent opening is inverted, in order to decrease light density at a boundary portion when the transmitted lights interfere with each other, thereby improving the resolution quality and focal depth of the transcription pattern. A photomask whose resolution quality has been improved by use of such a phase shift method is generally called a Levenson type phase shift mask.
Currently, the main method of providing a phase shift portion in one opening is a digging type method in which a transparent substrate is dug by, for example, etching to provide a shifter opening.
FIG. 8 is a schematic sectional view showing the structure of a dug-type Levenson type phase shift mask. In the drawing, the numeral 11 denotes a transparent substrate, 12 denotes a light shielding film, 13 denotes a non-shifter opening, and 14 denotes a shifter opening. The numeral 15 denotes a shallow trench, which is the depth of the non-shifter opening 13 dug in the substrate. The numeral 16 denotes an undercut, which is the length of a visor of light shielding portions provided in the shifter opening 14. The numeral 17 denotes a difference in digging amounts (or etching amounts) corresponding to a phase difference of 180° between transmitted light 3b through the non-shifter opening 13 and transmitted light 3a through the shifter opening 14. The numeral 18 denotes a chromium critical dimension (CD, refers to a line width, for example, in the case of an isolated pattern of a line pattern), which is a line width dimension when chromium (Cr) is used for the light shielding film 12. The numeral 19 denotes a pitch, which is a distance from an end face of a light shielding pattern to an end face of a next light shielding pattern.
In the Levenson type phase shift mask shown in FIG. 8, a structure without the shallow trench 15 is a so-called single trench structure, while a structure with the shallow trench 15 is a so-called dual trench structure. In both structures, it is known to provide the undercut 16 as described in, for example, Patent document 1 in order to prevent imbalance of light shielding intensity due to the transmitted light from a sidewall of a dug portion of the substrate.
The dug-type Levenson type phase shift mask described above is based on a structure in which the shifter openings and the non-shifter openings are alternately arranged in a repetitive manner. However, in an actual design of a device circuit, there are created not only a pattern in which the shifter openings and the non-shifter openings are alternately arranged but also a pattern in which the shifter openings adjoin each other or a pattern in which the non-shifter openings adjoin each other. For example, as shown in FIG. 9, there may be created a pattern in which a shifter opening 21a adjoins another shifter opening 22a. It is often impossible in a circuit design to avoid the creation of such a pattern in which the openings of the same kind adjoin each other. The pattern in which the openings of the same kind adjoin each other has the following two problems.
The first problem is that because the phases of transmitted lights 3a through the adjacent openings 21a and 22a are the same (π-π in the case of FIG. 9), they reinforce each other (not counteract each other), such that the light intensity in a portion 23a interposed between the openings of the same kind is increased, resulting in a degraded resolution quality of this interposed portion. FIG. 10 is a characteristic line diagram showing a relative exposure intensity when a positive resist is used to transfer a mask having the structure of FIG. 9 onto a semiconductor wafer. At the same threshold (SL), a resist CD 23b of the portion interposed between the shifter openings 21a and 22a of the same kind is smaller than resist CDs (transfer dimension of the resist) 25b and 26b of portions interposed between the shifter openings and the non-shifter openings, and the resolution is difficult in the former portion.
The second problem is that a light shielding film 12b present in the portion 23a interposed between the openings of the same kind easily comes off (or peels off). There is no problem when the non-shifter openings adjoin each other, but when the shifter openings 21a and 22a of the same kind adjoin each other as in the pattern shown in FIG. 9, the undercuts 16 are produced from both sides into the transparent substrate 11 in the portion 23a interposed between the two shifter openings 21a and 22a, such that a contact area between the light shielding film 12b and the transparent substrate 11 is reduced. Thus, the light shielding film 12b in the interposed portion 23a easily comes off from the transparent substrate 11. The mask design is greatly restricted due to countermeasures against the coming off of the light shielding film 12b, and the amount of the undercuts can not be properly selected, and moreover, the performance of the mask may deteriorate.
Patent document 1: Jpn. Pat. Appln. KOKAI Publication No. 10-333316